In a vehicle radio receiver included in radio receivers, to reduce the effects of the fading caused by the movement of a vehicle and secure the ability of the radio to receive analog broadcasting, diversity reception that selects an antenna which is in the best receiving condition from among a plurality of antennas is in widespread use.
However, the main reasons of the noise and the distortion of a received sound include multipath interference. The multipath interference is caused by the mutual interference of a plurality of signals propagating through different routes from a transmitter of a broadcasting station to reach an antenna. For example, a direct wave directly reaching thereto from a transmission antenna and some reflected waves are superimposed on each other, and the phase differences therebetween and the like extremely reduce the signal intensity of the received signal. Depending on the differences of path length between the received waves, if the difference of path length is small, a multipath interference wave having a short delay time reaches the antenna, and if the difference of path length is large, a multipath interference wave having a long delay time reaches thereto.
The level of multipath interference varies depending on geographical and building conditions. Particularly in an urban area where high-rise buildings stand side by side along both sides of a road, a broadcast wave propagates along a road, and is reflected by high-rise buildings to generate reflected waves having a short delay time. Therefore, when a large number of multistory buildings stand, a broadcast wave contains a lot of received signals with a short delay time.
Since a vehicle moves in such a changing radio wave environment during the running thereof, there may easily occur fluctuations in the electric field strength more frequently in a short time. Therefore, an antenna switching for selecting an antenna existing in the best receiving condition may easily occur frequently.
On the other hand, in recent years, terrestrial digital radio broadcasting is popularized, which simultaneously transmits analog and digital signals using the frequency of existing AM/FM analog broadcasting. In North America, about 1700 or more broadcasting stations have started the hybrid broadcasting by an IBOC (In-Band On-Channel) method, or are getting ready for sending. The hybrid broadcasting can use the same frequency as that of the existing analog broadcasting, different from the terrestrial digital radio broadcasting of Japan, and thus a broadcasting station need not afresh prepare a frequency for digital broadcasting. A radio receiver using a conventional analog method can receive existing broadcasting, and at the same time a hybrid radio receiver capable of receiving digital broadcasting can also receive digital one.
Incidentally, in a hybrid radio receiver capable of receiving the hybrid broadcasting using the above IBOC method, it is known that in performing antenna switching while receiving digital broadcasting, “out-of-synchronization” in digital demodulation occurs by the phase difference between the switched antennas. Particularly in an urban area, there frequently easily occurs the “out-of-synchronization” by the antenna switching.
In order to avoid the reduction of the ability for receiving the hybrid broadcasting using the above IBOC method due to the “out-of-synchronization,” an IBOC broadcasting receiver in a related art is known, which controls diversity reception to be disabled when it is judged that an IBOC broadcast wave exists based on a demodulation result (for example, see Patent Document 1).